Step a paused flow, and deliver what a rate limit held back

Three things a pause and an interval were quietly losing:

- A rebuild builds a fresh pipeline, so nothing is paused any more and no
  resume ever comes for what the old one parked. Release it on rebuild.
- POST /flows/{name}/step takes the oldest parked item and runs that one wave
  while the flow stays paused, so a held-back cascade can be walked through.
  Nothing parked answers plainly rather than failing.
- A per-port interval was leading-edge only: a producer going quiet inside the
  window left the consumer on the value before it. The held value is kept and
  a flush item scheduled on the queue's existing timer, so the window ends with
  a delivery. One timer in flight per node, and none without a queue to run it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

backend/app/api/routes/flows.py

@@ -541,6 +541,16 @@ async def resume_flow(name: str, controller: FlowControllerDep) -> Any:
     return Message(message=f"Resumed flow '{name}'")
 
 
+@router.post("/{name}/step", response_model=Message)
+async def step_flow(name: str, controller: FlowControllerDep) -> Any:
+    """Run one message a pause is holding back, leaving the flow paused."""
+    _read_flow(controller, name)
+    node = await run_in_threadpool(controller.step_flow, name)
+    if node is None:
+        return Message(message=f"Nothing held back in flow '{name}'")
+    return Message(message=f"Stepped '{node}'")
+
+
 # -----------------------------------------------------------------------------
 # Validation and execution
 # -----------------------------------------------------------------------------